The present invention relates to the hydrogenation of coal in general, and more particularly to a method of increasing the oil yield from such coal hydrogenation.
Coal oil is usually obtained from coal by subjecting finely comminuted coal particles to a hydrogenation operation at an elevated pressure and at a high temperature by contacting the coal particles with molecular hydrogen or with hydrogen-releasing solvents. During this operation, the coal which, among others, contains high-molecular aromatic compounds, is decomposed, at least to the predominant degree, into low-molecular products. These low-molecular products include gaseous hydrocarbons, distillable oils, as well as soluble and meltable, but not distillable substances which can be found in the so-called hydrogenation residue.
While the liquid fractions or oils can be used in motor vehicle fuels or as raw materials in the chemical industry, the gaseous hydrocarbons can be used as gaseous fuels or also as starting materials in the chemical industry. On the other hand, the hydrogenation residue is either returned into the hydrogenation operation, which may result in difficulties in the processing, or it may be subjected to a low-temperature carbonization which, however, results in a considerable burden on the environment.
The hydrogenation residue consists of non-reacted coal, mineral components of the coal, asphalts, as well as of minute amounts of heavy oils. According to the current concepts in the area of large-scale coal hydrogenation, the components of the residue should be gasified in order to produce in this manner a part of the hydrogen needed for the coal hydrogenation.
It is known that asphalt-containing hydrogenation products can be decomposed into distillable oils in a stationary bed reaction vessel on catalysts selected from the series of transition metals, under hydrogen pressure and at high temperatures, at least to a considerable degree. However, this process, which is known, for instance, from "Chemieprodukte aus Kohle" by Falbe, published by Verlag Thieme in Stuttgart in 1977, page 98, requires a preceding segregation of the insoluble mineral and coal components by filtration. This is a very cumbersome and difficult operation which is too expensive to be used for the required high throughputs. In addition thereto, the catalysts become inactive within a short period of time due to the deposition of coke particles.
It is further known that asphalt-containing hydrogenation products can be decomposed into distillable oils in high boiling point aromatic oils and with the addition of catalysts at temperatures between 450.degree. and 500.degree. C., as disclosed in the above publication on page 84. The throughput, however, is only sufficient when highly active catalysts are being used, or when the operation is conducted at very high pressures of up to 700 bar.
In these conventional methods, the catalysts can not be reclaimed. Additionally, the solvent is partially decomposed into gaseous hydrocarbons, so that both of the conventional methods are performed under very uneconomical conditions.